The art generally acknowledges the potential utility for microporous materials made from ethylene chlorotrifluoroethylene (“ECTFE”) copolymer, a solvent-resistant material. Microporous materials can generally be fabricated using a phase separation process such as “thermally induced phase separation” (“TIPS”). But, a TIPS process has not been used successfully in the fabrication of ECTFE microporous materials that are also capable of enduring further processing such as the removal of diluent and stretching of the material to impart a high degree of porosity. ECTFE membranes have typically lacked the strength needed to withstand being folded and pleated in the manner required to create high surface area filtering materials that are suitable for placement in a filter cartridge, for example.
Nucleating agents have been used in the preparation of microporous materials. While certain materials have been successfully employed as nucleating agents in various polymer systems, known nucleating agents have failed to provide ECTFE microporous materials with a desired morphology that is strong enough to withstand further processing (e.g., stretching, folding).
Methods for introducing nucleating agents into a polymer/diluent system include ‘pre-mixing’ methods that first require the preparation of a dispersion of nucleating agent in a diluent, typically using a high shear mixer. Alternatively, nucleating agent may first be dispersed in the polymer to make a compounded ‘masterbatch.’ But, when employed in a TIPS process for the manufacture of ECTFE microporous materials by extrusion, nucleating agents often agglomerate and/or fall out of the solution/dispersion while travelling through the extrusion system. Pre-mixing techniques have generally failed to provide either a sufficiently rapid rate of ECTFE crystallization or a desired morphology (e.g., a spherulitic matrix).